Optical information recording medium

ABSTRACT

An optical information recording medium of this invention includes a substrate, a light incident surface, a first reflecting layer formed between the substrate and the light incident surface, a second reflecting layer formed between the first reflecting layer and the light incident surface and stacked on the first reflecting layer, the second reflecting layer being made of the same material as that of the first reflecting layer, and a phase change optical recording layer formed between the second reflecting layer and the light incident surface, the phase change optical recording layer transiting between a crystal state and an amorphous state when irradiated with a light beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2001-359607, filed Nov.26, 2001, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical information recordingmedium capable of recording and reproducing large-capacity informationusing a light beam.

[0004] 2. Description of the Related Art

[0005] Recently, in the field of information recording, research intooptical information recording media and optical information recordingmethods are progressing at various laboratories. Optical informationrecording media can record/reproduce information in a noncontact state.Optical information recording media are classified into read-only-type,write-once, read-many-type, and rewritable media and can cope withvarious memory forms. Such optical information recording media caninexpensively store large-capacity files and are expected to be widelyused as industrial and consumer devices.

[0006] CDs, LDs, and DVDs (Digital Versatile Discs) corresponding to aread-only memory form have already been widely proliferated. Theseoptical disks have a transparent substrate on which a three-dimensionalpattern such as pits and grooves that indicate an information signal isformed. A reflecting film formed from a metal thin film of, e.g.,aluminum is formed on the transparent substrate. A protective film forprotecting the reflecting film from oxidation is formed on thereflecting film. A light beam incident on the optical disk is reflectedby the reflecting film. The three-dimensional pattern such as pits andgrooves that indicate an information signal is reflected on reflectedlight reflected by the reflecting film. Hence, when a change inreflected light is detected, the information signal can be reproduced.

[0007] Phase change optical disks corresponding to a rewritable memoryform are already forming a market of PDs, DVD-RAMs, and DVD-RWs. Thedisk structure will be described below. A transparent dielectric film isformed on a transparent substrate. A phase change recording layeressentially consisting of Ge, Sb, Te, In, or Ag is formed on thetransparent dielectric film. Another transparent dielectric film isformed on the phase change recording layer. A reflecting film made of,e.g., aluminum is formed on the transparent dielectric film. Inaddition, a protective film made of, e.g., a UV curing resin is formedon the reflecting film. Upon receiving a light beam from a semiconductorlaser, the phase change recording layer on the transparent substratereversibly transits between an amorphous state and a crystal state. Inan information reproduction mode, an information signal is reproduced bydetecting a change in reflected light from a recording portion of thephase change recording layer. In an information recording mode, arecording portion of the phase change recording layer is irradiated witha short-pulse light beam having a relatively high power to heat therecording portion to a temperature equal to or more than the meltingpoint. Then, the recording portion is quickly cooled to form anamorphous recording mark at the recording portion. In an informationerase mode, the recording portion of the phase change recording layer isirradiated with a long-pulse light beam having a lower power than in therecording mode to hold the recording portion at a temperature betweenthe crystallization temperature (inclusive) and the melting point(exclusive) or cool the recording portion from a temperature equal to ormore than the melting point, thereby crystallizing the recordingportion. As described above, in the phase change optical recording,information is recorded using a change in reflectance between theamorphous state and the crystal state. For this reason, an apparatus canhave an optical system with a simple structure. Phase change opticalrecording requires no magnetic field, unlike magnetooptical recording.Additionally, in phase change optical recording, an overwrite by lightintensity modulation is easy, and the data transfer rate is high.Furthermore, phase change optical recording has good compatibility witha read-only disk represented by a DVD-ROM and CD-ROM.

[0008] As a method of increasing the capacity of such an optical disk,the NA (Numerical Aperture) of the objective lens of an optical pickupis increased to reduce the spot diameter of reproduction light, therebyattaining a high recording density. In a shift from, e.g., a CD to aDVD, the substrate thickness is decreased from 1.2 mm to 0.6 mm to copewith an optical system with a high NA. To increase the NA, thetransparent substrate through which reproduction light passes must bemade thinner. This is because when the NA is increased, the allowableamount of aberration generated by the angle of shift of the disk surfacefrom a plane perpendicular to the optical axis of the optical pickup,i.e., the tilt angle becomes small. For this reason, as the NAincreases, the transparent substrate must be made thin, and thesubstrate thickness distribution in the disk must fall within apredetermined range.

[0009] For a recording/reproduction optical disk such as a DVD, a lightbeam becomes incident from the substrate side. That is, a light beamirradiation surface in the reflecting layer is formed on the protectivelayer. An interface is formed between the reflecting layer and theprotective layer. Since the surface of the protective layer is reflectedon the light beam irradiation surface in the reflecting layer, anequilibrium is maintained. On the other hand, in a high-NA-compatibleoptical disk which is applied to an apparatus having an optical pickupwith a high-NA lens, layers are formed in an order reverse to that ofthe above-described conventional optical disk to ensure a tilt margin.In such a high-NA-compatible optical disk, a reflecting layer, secondprotective layer, phase change recording layer, and first protectivelayer are formed in this order. For this reason, the surface state ofthe reflecting layer is reflected on the second protective layer formedon the reflecting layer, the recording layer formed on the secondprotective layer, and the first protective layer formed on the recordinglayer. Generally, crystal grains on the surface of the reflecting layerformed from an AL alloy tend to have a large side due to columnar growthunique to a metal thin film. The surface roughness of the reflectinglayer also roughens the surface of the recording layer through thesecond protective layer. A mark recorded on the high-NA-compatibleoptical disk which aims at increasing density by increasing the NA isfiner than a mark recorded on the conventional optical disk. That is,the above-described surface roughness of the recording layer greatlyinfluences the recording/reproduction characteristic of thehigh-NA-compatible optical disk. More specifically, the surfaceroughness of the recording layer produces noise in the reproduction modeor causes strain at a mark edge in forming a recording mark. Hence, in ahigh-NA-compatible optical disk, such surface roughness(three-dimensional pattern) of the reflecting layer is preferablysuppressed.

[0010] In the conventional optical disk in which a light beam isincident from the substrate surface side, a reflecting layer isdivisionally formed for efficient mass production. With the divisionalreflecting layer formation, the columnar growth of the reflecting layeris slightly suppressed. However, the surface roughness of the reflectinglayer generates noise in the reproduction mode or fluctuates a mark edgeat a {fraction (1/10)} wavelength or more. For this reason, even theabove-described columnar growth suppression by divisional film formationdoes not suffice in obtaining a satisfactory recording/reproductioncharacteristic. In the conventional optical disk, an Al-based materialis used for the reflecting layer. However, this material readily formslarge-size crystal grains and is therefore unsuitable for ahigh-NA-compatible optical disk in which layers are formed in a reverseorder. As a method of reducing the surface roughness of the recordinglayer of a high-NA-compatible optical disk, a method of inserting ametal undercoat between the reflecting layer and the substrate isproposed in Jpn. Pat. Appln. KOKAI Publication No. 11-327890. However,this method has another problem that the disk manufacturing costincreases because an additional layer is formed.

BRIEF SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide an opticalinformation recording medium having an excellent recordingcharacteristic.

[0012] According to an embodiment of the present invention, there isprovided an optical information recording medium comprising a substrate,a light incident surface, a first reflecting layer formed between thesubstrate and the light incident surface, a second reflecting layerformed between the first reflecting layer and the light incident surfaceand stacked on the first reflecting layer, the second reflecting layerbeing made of the same material as that of the first reflecting layer,and a phase change optical recording layer formed between the secondreflecting layer and the light incident surface, the phase changeoptical recording layer transiting between a crystal state and anamorphous state when irradiated with a light beam.

[0013] Additional objects and advantages of the present invention willbe set forth in the description which follows, and in part will beobvious from the description, or may be learned by practice of thepresent invention. The objects and advantages of the invention may berealized and obtained by means of the instrumentalities and combinationsparticularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0014] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate embodiments of thepresent invention and, together with the general description given aboveand the detailed description of the embodiments given below, serve toexplain the principles of the present invention.

[0015]FIG. 1 is a sectional view showing the layer structure of a disk Aas an optical information recording medium according to the embodimentof the present invention;

[0016]FIG. 2 is a sectional view showing the layer structure of a diskB;

[0017]FIG. 3 is a view showing the AFM image of the recording layersurface of a sample a2;

[0018]FIG. 4 is a view showing the AFM image of the recording layersurface of a sample b2;

[0019]FIG. 5 is a graph showing changes in CNR of closest patterns 3Tdepending on the repetitive numbers of times of recording of the disks Aand B;

[0020]FIG. 6 is a graph showing changes in jitter depending on therepetitive numbers of times of recording of the disks A and B;

[0021]FIG. 7 is a graph showing a change in jitter depending on therepetitive number of times of recording of a disk obtained by replacingthe AlTi reflecting layers of the disk A with AgPdCu reflecting layers;

[0022]FIG. 8 is a graph showing changes in jitter in a disk obtained byreplacing the AlTi reflecting layers of the disk A with an AlMoreflecting layer and, more specifically, the relationship between Ra/t(t is the recording layer thickness) and the jitter in the first-timerecording and the relationship between Ra/t and the jitter afterrecording was performed 10,000 times;

[0023]FIG. 9 is a sectional view showing the layer structure of a diskhaving three protective layers; and

[0024]FIG. 10 is a sectional view showing the layer structure of a diskhaving two recording layers.

DETAILED DESCRIPTION OF THE INVENTION

[0025] An embodiment of the present invention will be described below.

[0026] In an optical information recording medium according to anembodiment of the present invention, a light incident surface,transparent cover sheet, protective layer, phase change recording layer(to be referred to as a recording layer hereinafter), reflecting layer,and substrate are formed sequentially from one surface. That is, thereflecting layer is formed on the substrate. A light beam becomesincident from the opposite side of the reflecting layer with respect tothe substrate. As the most important point, the reflecting layer isformed from first and second reflecting layers. The first and secondreflecting layers are made of the same material.

[0027] For the above-described recording layer, a material whichtransits between a crystal state and an amorphous state upon receiving alight beam and exhibits different optical characteristics between thetwo states is used. An example of this material is a ternary materialsuch as Ge—Sb—Te or In—Sb—Te. Even when at least one of Co, Pt, Pd, Au,Ag, Ir, Nb, Ta, V, W, Ti, Cr, Zr, Bi, Sn, and the like is added to thematerial in a very small amount, the recording layer can obtain asatisfactory characteristic. To realize a satisfactory recording/erasecharacteristic, the thickness of the recording layer is preferably 10 to20 nm. An average surface roughness Ra is preferably {fraction (1/50)}to {fraction (1/10)} of the recording layer thickness. When the surfaceroughness Ra is {fraction (1/50)}, the surface energy becomes small, andthe adhesion between the recording layer and a layer stacked on itdecreases. Since the decrease in adhesion deteriorates the repetitiverecording characteristic and shortens the storage life, a surfaceroughness Ra of {fraction (1/50)} or less is unsuitable. When thesurface roughness Ra is {fraction (1/10)} or more, a change in lightabsorbance along with a local change in film thickness of the recordinglayer cannot be neglected. This is undesirable because it causes strainin the recording mark edge and lowers the jitter. With the abovestructure, the recording layer can have a satisfactory characteristic.In addition, the reflectance when the recording layer is in the crystalstate becomes lower than that in the amorphous state.

[0028] The protective layer in the optical information recording mediumaccording to the embodiment of the present invention mechanically andchemically protects the phase change recording layer. It also serves asan interference film for adjusting the optical characteristic of theoptical information recording medium. As the protective layer, atransparent dielectric film having a refractive index of 2.0 to 3.5 andan extinction coefficient of 0 to 0.2 is preferably used. A protectivelayer material preferably contains at least one of, e.g., Zn—S, Si—O,Si—N, Ti—O. Ge—N, Ta—O, Al—N, Cr—O, and SiC. Especially, a compositematerial containing Si—O and Zn—S is preferable. These protective layermaterials can also have an effect for promoting crystallization of therecording layer. Generally, since the stoichiometric ratio of a thinfilm material largely changes depending on the process condition, theratio of component elements is not specified.

[0029] The reflecting layer in the optical information recording mediumaccording to the embodiment of the present invention is indispensable toreflect an irradiation light beam and efficiently use the opticalenergy. The reflecting layer also has an effect of a heat dissipationlayer in controlling the heating/cooling process unique to the phasechange recording medium.

[0030] The reflecting layer in the optical information recording mediumaccording to the embodiment of the present invention is formed fromfirst and second reflecting layers made of the same material to make thesurface roughness as small as possible. The first reflecting layer isformed on the substrate. After a predetermined time, the secondreflecting layer made of the same material as that of the firstreflecting layer is formed on the first reflecting layer. Since thinfilm formation is temporarily stopped between the first and secondreflecting layers to stop crystal growth, growth of any large-sizegrains can be suppressed. At this time, a time of several secondssuffices between formation of the first reflecting layer and formationof the second reflecting layer. The time is preferably 5 seconds ormore. In dividing the reflecting layer, when the number of divisions ischanged in accordance with the material of the reflecting layer, thesurface roughness can be controlled in more detail. For a material suchas an Al-based material which readily causes large-size grain growth,the surface roughness reduction effect can be increased by dividing thereflecting layer into three or four layers. On the other hand, for amaterial such as Ag-based material with relatively small crystal grains,a sufficient effect can be obtained even by two layers. The surfaceroughnesses of the protective layer and recording layer formed on thereflecting layer having such a multi-layered structure are essentiallysmall. Hence, the recording/reproduction characteristic represented by ajitter characteristic considerably improves. As described above, only bydivisionally forming the reflecting layers in a plurality of steps (onlyby interrupting the reflecting layer formation process), the roughnessesof the surfaces of the reflecting layer and recording layer can besuppressed. That is, the recording/reproduction characteristic can beimproved without increasing the manufacturing cost of the opticalinformation recording medium. The reflecting layer material ispreferably an alloy mainly containing Ag, Al, or Au and more preferablyan Ag alloy that contains an additive element in 5 at % or less and hasan excellent resistance against an environment. The reflecting layer inthe optical information recording medium of the present inventionpreferably has a total thickness of 30 to 200 nm.

[0031] The optical information recording medium according to theembodiment of the present invention can easily be discriminated byobserving its sectional shape with an electron microscope or the like.For example, when an optical information recording medium using anAl-based reflecting layer according to the present invention is observedwith a transmission electron microscope, the first and Al-basedreflecting layers are separated from each other.

[0032] The respective layers according to the embodiment of the opticalinformation recording medium of the present invention can be formed by ageneral physical deposition method. The layers can be formed by any filmformation method such as RF/DC sputtering, electron beam deposition,resistance heating deposition, or molecular beam epitaxy (MBE). In sucha thin film forming method represented by RF sputtering, the filmcharacteristic changes according to the process condition in filmformation. For example, a reflecting layer with a small surfaceroughness is preferably formed at a high growth rate. According to theoptical information recording medium of the present invention, thesurface roughnesses of the reflecting layer and recording layer can bereduced without forming any additional layer. Hence, the diskperformance can be expected to improve.

[0033] Example 1 of the optical information recording medium of thepresent invention will be described below.

[0034] The interior of a film forming apparatus was exhausted to 5×10⁻⁴(Pa) or less. A DC power of 1 kW was applied to an AlTi target to form a50-nm thick first reflecting layer on a 1.1-mm thick PC substrate. Tenseconds after that, film formation was resumed to form a 50-nm secondreflecting layer. That is, AlTi reflecting layers having a totalthickness of 100 nm were formed. After that, an information layer wasformed by RF magnetron sputtering. That is, an RF power of 1 kW wasapplied to a ZnS(80)-SiO₂(20) target to form a 15-nm protective layer.Subsequently, an RF power of 500 W was applied to a Ge target in a gasmixture atmosphere containing argon and nitrogen to form a 5-nm thickGeN layer. Next, an RF power of 250 W was applied to a Ge₄₀Sb₈Te₅₂target to form a 15-nm thick phase change recording layer. Subsequently,an RF power of 500 W was applied to a Ge target in a gas mixtureatmosphere containing argon and nitrogen to form a 5-nm thick GeN layer.Next, an RF power of 1 kW was applied to a ZnS(80)-SiO₂(20) target toform a 50-nm thick protective layer. After that, theinformation-layer-side surface of the substrate extracted from the filmforming apparatus was coated with a UV curing resin. Then, a 0.1-nmthick polycarbonate (PC) cover layer was bonded. The resultant structurewas rotated by a spinner to decrease the resin thickness to several μmand irradiated with UV light to cure the resin layer. In this way, adisk A as the optical information recording medium of the presentinvention was completed. A disk B was prepared by continuously formingAlTi reflecting layers having a total thickness of 100 nm and formingthe remaining films under the same conditions and compared with the diskA.

[0035]FIG. 1 is a sectional view showing the layer structure of the diskA as the optical information recording medium of the present invention.As shown in FIG. 1, a first AlTi reflecting layer 12 a, second AlTireflecting layer 12 b, ZnS(80)-SiO₂(20) protective layer 13, GeN layer14, Ge₄₀Sb₈Te₅₂ recording layer 15, Ge layer 16, ZnS(80)-SiO₂(20)protective layer 17, UV curing resin layer 18, and a 0.1-mm thick PCsheet 19 were sequentially stacked on a 1.1-mm thick PC substrate 11 ofthe disk A. Since the second reflecting layer 12 b was formed after theelapse of a predetermined time from formation of the first reflectinglayer 12 a, a boundary 12 c was present between the first reflectinglayer 12 a and the second reflecting layer 12 b. This disk A isirradiated with a light beam from the PC sheet 19 side. That is, theupper surface of the PC sheet 19 serves as a light incident surface 10.

[0036]FIG. 2 is a sectional view showing the layer structure of the diskB. As shown in FIG. 2, an AlTi reflecting layer 22, ZnS(80)-SiO₂(20)protective layer 23, GeN layer 24, Ge₄₀Sb₈TeS₂ recording layer 25, GeNlayer 26, ZnS(80)-SiO₂(20) protective layer 27, UV curing resin layer28, and 0.1-mm thick PC sheet 29 were sequentially stacked on a 1.1-mmthick PC substrate 21 of the disk B. The reflecting layer 22 in the diskB had no boundary.

[0037] AlTi single layer samples a1 and b1 formed on Si substrates inaccordance with the same procedure as that for the disks A and B andsamples a2 and b2 for which a structure from an AlTi reflecting layer toa Ge₄₀Sb₈Te₅₂ recording layer were formed were prepared.

[0038] The AlTi single layer samples a1 and b1 were observed with atransmission electron microscope (TEM). The crystal grain distributionin a plane was observed. The average grain diameter was 34.2 nm for thesample a1 and 51.6 nm for the sample b1. The sections of the sampleswere observed with the TEM. For the sample a1, a boundary formed byinterrupted film formation was clearly observed. This revealed thatcrystal growth was suppressed. For the sample b1, crystal growth for thesubstrate reached the surface layer, and a large-size crystal wasformed.

[0039] The recording layer surfaces of the samples a2 and b2 wereobserved with an atomic force microscope (AFM). FIG. 3 is a view showingthe AFM image of the recording layer surface of the sample a2. FIG. 4 isa view showing the AFM image of the recording layer surface of thesample b2. The surface of the sample a2 was smooth relative to that ofthe sample b2. The average surface roughness Ra was 2.7 nm for thesample b2 but 0.8 nm for the sample a2, which exhibited a verysatisfactory smoothness. The surface roughness Ra of the sample b2 was{fraction (1/10)} or more of the recording layer thickness, 15 nm.However, the surface roughness Ra of the sample a2 was {fraction (1/10)}or less of the recording layer thickness, 15 nm. The influence of thedifference in medium structure on the recording/reproductioncharacteristic was examined. The recording/reproduction characteristicsof the disks were evaluated under the following conditions. Laser output0.1 to 6.0 mW Light source wavelength 405 nm Disk rotational speed 6.0m/s Objective lens NA 0.85 Track pitch 0.30 μm Shortest bit length 0.12μm Recording pattern 3T, 11T Modulation scheme (8, 16) RLL

[0040]FIG. 5 is a graph showing changes in CNR of closest patterns 3Tdepending on the repetitive numbers of times of recording of the disks Aand B. The CNR of the disk A as the optical information recording mediumof the present invention maintained a high level of 52 dB or more for10,000 times. To the contrary, the CNR of the disk B remained 49 dB orless. The CNR of the disk A was high mainly because of low noise. Thisreflected that the AlTi reflecting layer had fine crystal grains, i.e.,the surface roughness of the recording layer was small.

[0041]FIG. 6 is a graph showing changes in jitter depending on therepetitive numbers of times of recording of the disks A and B. Thedifference between the disks can be seen from the jitter characteristicshown in FIG. 6. The disk A has a satisfactory jitter value of about 8%while the jitter of the disk B exceeds 11%.

[0042] Comparison between the disks A and B proved that the disk A asthe optical information recording medium of the present invention had asmall three-dimensional pattern on the recording layer and exhibited anexcellent jitter characteristic.

[0043] Example 2 of the optical information recording medium of thepresent invention will be described next.

[0044] An optical disk was manufactured in accordance with the sameprocedure and layer structure as in the disk A of Example 1 except thatthe reflecting layers (first AlTi reflecting layer 12 a and second AlTireflecting layer 12 b) were changed to two 25-nm thick AgPdCu layers,i.e., layers having a total thickness of 50 nm. FIG. 7 is a graphshowing a change in jitter depending on the repetitive number of timesof recording of the disk obtained by replacing the AlTi reflectinglayers of the disk A with the AgPdCu reflecting layers. Due to theeffect of the AgPdCu reflecting layers having a smaller grain diameterthan the AlTi reflecting layers, the jitter characteristic became muchbetter than disk A. A jitter of 8% or less was obtained in repetitiverecording of 10,000 times.

[0045] Example 3 of the optical information recording medium of thepresent invention will be described next.

[0046] An optical disk was manufactured in accordance with the sameprocedure and layer structure as in the disk A of Example 1 except thatthe reflecting layers (first AlTi reflecting layer 12 a and second AlTireflecting layer 12 b) were changed to an AlMo reflecting layer. Asurface roughness Ra of the recording layer was changed by appropriatelychanging the number of divisions of the AlMo reflecting layer. Thesurface roughness Ra was obtained through AFM observation. FIG. 8 is agraph showing changes in jitter in the disk obtained by replacing theAlTi reflecting layers of the disk A with the AlMo reflecting layer and,more specifically, the relationship between Ra/t (t is the recordinglayer thickness) and the jitter in the first-time recording and therelationship between Ra/t and the jitter after recording was performed10,000 times. The jitter in the first-time recording exhibited asatisfactory characteristic of about 8% when Ra/t≦0.1. To the contrary,the jitter after recording was executed 10,000 times exhibited anincrease not only when Ra/t>0.1 but also when Ra/t<0.02. That is, adegradation in disk performance was observed. As can be seen from theabove results, the ratio of the surface roughness Ra to the recordinglayer thickness t was appropriately 0.02 to 0.1. As described above, theeffect of the present invention was confirmed even in a disk having adifferent reflectance polarity.

[0047] Example 4 of the optical information recording medium of thepresent invention will be described next.

[0048] As shown in FIG. 9, a disk was manufactured in which AlMo (50nm*3) (i.e., AlMo (50 nm) reflecting layer 32 a, AlMo (50 nm) reflectinglayer 32 b, and AlMo (50 nm) reflecting layer 32 c), ZnS—SiO₂ (15 nm)protective layer 33, GeN (5 nm) layer 34, Ge₃₀Sb₁₆Te₅₄ (13 nm) recordinglayer 35, GeN (5 nm) layer 36, ZnS—SiO₂ (20 nm) protective layer 37,SiO₂ (60 nm) protective layer 38, ZnS—SiO₂ (30 nm) protective layer 39,UV curing resin layer 40, and PC sheet 41 were sequentially stacked on a1.1-mm thick PC substrate 31. This disk is irradiated with a light beamfrom the PC sheet 41 side. That is, the upper surface of the PC sheet 41serves as a light incident surface 30. The AlMo reflecting layer wasdivided into three layers (32 a, 32 b, and 32 c) at a wait time of,e.g., 6 seconds. AlMo reflecting layers having a total thickness of 150nm were formed. Section TEM observation showed that the grain growth wasinterrupted, and two boundaries 32 d and 32 e were present between thedivided portions schematically shown in FIG. 9. The jittercharacteristic was evaluated, as in the above examples. The jitterexhibited a satisfactory characteristic that fell within the range of7.5% to 8.5% for repetitive recording of 10,000 times or more. InExample 4, the reflectance before recording was about 5%, i.e., lowerthan that after recording. When the reflectance is low, focusing ortracking readily becomes difficult. Especially in a high-NA-compatiblemedium, this tendency gets stronger by the three-dimensional pattern onthe reflecting layer. When the reflecting layer was divided, as inExample 4, stable tracking was obtained even in a disk having such areflectance polarity.

[0049] Example 5 of the optical information recording medium of thepresent invention will be described next.

[0050] As shown in FIG. 10, AlTi (50 nm*2) (i.e., AlTi (50 nm)reflecting layer 52 a and AlTi (50 nm) reflecting layer 52 b), ZnS—SiO₂(30 nm) protective layer 53, GeN (3 nm) layer 54, second Ge₂Sb₂Te₅ (13nm) recording layer 55, GeN (3 nm) layer 56, and ZnS—SiO₂ (90 nm)protective layer 57 were sequentially formed on a 1.1-mm thick PCsubstrate 51 by sputtering. The AlTi reflecting layer was divided intotwo layers (52 a and 52 b) at a wait time of, e.g., 8 sec. AlTireflecting layers having a total thickness of 100 nm were formed.Parallelly, a ZnS—SiO₂ (30 nm) protective layer 64, GeN (2 nm) layer 63,first Ge₄₀Sb₈Te₅₂ (6.5 nm) recording layer 62, GeN (2 nm) layer 61,ZnS—SiO₂ (25 nm) protective layer 60, and AgPdCu (10 nm) layer 59 weresequentially formed by sputtering on a 0.1-mm thick PC sheet 65 havingthe same track shape as that of the PC substrate 51. The film surfacesof the two media were bonded via a 30-μm thick UV curing resin layer 58.In such an optical disk having two recording layers, a light beamincident from a light incident surface 50 is independently focused onthe first and second recording layers to allow recording/reproduction onthe respective layers. Hence, the capacity of the disk can be increased.Section TEM observation showed that the grain growth was interrupted,and a boundary 52 c was present between the AlTi reflecting layersschematically shown in FIG. 10. The jitter characteristic was evaluated.The jitter exhibited a satisfactory characteristic that fell within therange of 8.5% to 9.0% in the first recording layer and the range of 7.5%to 8.0% in the second recording layer for repetitive recording of 10,000times or more.

[0051] Especially in the second recording layer, a smaller jitter wasobtained, and the effect was confirmed.

[0052] As has been described above in detail, an excellent jittercharacteristic can be obtained using the optical information recordingmedium according to the present invention.

[0053] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. An optical information recording mediumcomprising: a substrate; a light incident surface; a first reflectinglayer formed between said substrate and said light incident surface; asecond reflecting layer formed between said first reflecting layer andsaid light incident surface and stacked on said first reflecting layer,said second reflecting layer being made of the same material as that ofsaid first reflecting layer; and a phase change optical recording layerformed between said second reflecting layer and said light incidentsurface, said phase change optical recording layer transiting between acrystal state and an amorphous state when irradiated with a light beam.2. A medium according to claim 1, wherein an average surface roughnessRa of said recording layer is {fraction (1/50)} to {fraction (1/10)}(both inclusive) of a thickness of said recording layer.
 3. A mediumaccording to claim 1, wherein said first and second reflecting layerscontain an Ag alloy.
 4. A medium according to claim 1, wherein areflectance when said recording layer is in the crystal state is lowerthan that when said recording layer is in the amorphous state.
 5. Amedium according to claim 4, further comprising a protective layerformed between said phase change recording layer and said light incidentsurface and formed from three layers.
 6. A medium according to claim 1,wherein said phase change recording layer includes a plurality ofrecording layers capable of independently recording/reproduction.